1. Field of the Invention
The present invention generally relates to electron emission devices. More particularly, it relates to a method and apparatus for improving the performance of field emitter devices by detecting the emission of electrons at excessively positive potentials and regulating the current produced at the excessively positive potentials.
2. Description of Related Art
Field emission is a tunneling process where electrons move from a solid, through a thin potential barrier, into vacuum without changing energy. The field emitted current increases as a function of the electric field at the emitter surface. A macroscopic field emitter tip requires a voltage typically greater than 100V and often more than 1000V to cause emission. The electronic, chemical, and geometric properties of the emitter surface also have a substantial effect on the field emission current. These properties can change as a result of field emission, especially when adsorbed and reacted atoms are present on the emitting surface.
When electrons tunnel from states at energies below EF, electronic energy is released as the empty state is filled. If this electronic energy is large enough and is directly coupled to chemical bonds, the bonds can break, thus releasing atoms to the vacuum, stimulating atomic motion on the surface, and/or causing chemical reactions. Chemical bonds typically have energies of 2-5 eV, so emission from energies more than 2 eV below EF can potentially stimulate these changes.
A similar electronic mechanism occurs when positive charge and very high local electric fields are created as electrons tunnel out of insulating or semi-conducting material. If the electric fields become too high, local breakdown may result. The field emission characteristics of each emitter typically change continuously during operation as a result of this electronic excitation. If atoms are released into vacuum as a result, arcs can occur at the field emission site.
Such arcs release the energy stored in the charged capacitance formed by the high voltage emitter, potentially causing significant physical damage. Although the probability of direct coupling to a bond may be low, it is possible that filling a single low energy state could break a bond. In contrast, large current densities are typically required to heat the emitter to a point where bonds may be broken. Thus, the emission current required to cause such thermal effects is often much higher than the currents at which failures are found to occur.
The electronic energy released after tunneling increases as the energy of the initial state becomes more positive (lower electron energy). Cleaning a metallic emitter surface of foreign atoms (and keeping it clean during operation) typically reduces the low energy emission and increases the maximum emission current which can be produced without causing an arc. Cleaning can be accomplished by heating the emitter to very high temperature in ultra high vacuum or by applying a very large negative electric field so as to field-desorb the surface atoms.
Cleaning may also occur spontaneously during emission because of the electronically-stimulated reactions mentioned above, or due to bombardment by ions created by the emitted electrons. However, subsequent contamination generally occurs within a few hours or minutes even when the emitter is maintained in ultra-high vacuum.
Field emitter arrays are micro-fabricated arrays of many small field emission structures (cells) and are known in the art. Each individual cell includes an emission site on the substrate and an aperture in a conducting layer (called the gate) deposited over a dielectric layer. The size of the apertures is typically about 1 micron, but may be much smaller. The distance between cells is typically 3-4 times the aperture diameter, but may be larger. A large electric field is created at the emission site when a positive voltage is applied to the gate with respect to the emitter.
An FEA typically requires an emitter-gate voltage of at least 10V and sometimes more than 100V to cause emission. In many applications, operation of the arrays must occur in relatively poor vacuum, and most arrays cannot be heated to temperatures high enough to remove adsorbed surface atoms. Thus, emission typically occurs from surfaces covered with adsorbed atoms, and the electronic properties of the adsorbed atoms frequency dominate the emission properties.
Because the area of a single cell is small compared to the area required to make an external connection, only a limited number of connections to the array are practical. Thus, in typical state of the art arrays a large number of cells (xcx9c10,000) share the same electric connection to an external voltage source.
Ideally, the field emitter arrays would be able to provide total currents nearly equal to the number of cells in the array multiplied by the current a single cell can produce. However, the cells typically do not have uniform emission properties and will fail if the emission current is excessive. Thus, only a small number of cells contribute to the emission current, so the arrays do not produce nearly as much current as they might if the emitters were more uniform.
The emission current can also vary with time and from place to place over the array as a result of spatial and temporal non-uniformity in the physical and chemical properties of the emitting sites. This variation is undesirable for many applications.
One known method of forcing the emission currents from each of the individual cells in an array to be more equal is to place a current-limiting circuit element, typically a resistance, in series with each emitter. If the resistances are large enough, the voltages developed across the resistors dominate the emission properties of each cell. Thus, the emission current can be nearly as uniform from cell to cell as are the resistances. This sort of scheme is workable for some applications such as displays requiring relatively small current densities and frequencies.
However, the voltages developed across the resistors change the energy of the emitted electrons, increasing the energy distribution (energy spread) of the beam, which is undesirable for many applications. The resistors also reduce the transconductance (dI/dV) and frequency response of the arrays. Although more complex current-limiting circuits can reduce such problems, any circuit that changes the potential of the emission site will increase the energy spread of the emitted electron beam.
FIG. 1 shows a cross sectional view of a single cell within a prior art field emitter array (FEA). Although a single cell is shown herein for the sake of simplicity, an overall FEA includes many of these cells, fabricated in a planar array. An emitter structure 3 is created on a conductive substrate 2 (or a conductive layer on an insulating substrate) in such a way that when a voltage source 12 is connected between the conductive gate layer 8 and the substrate 2, a field emission current is induced at the emission sites 4 of the emitter 3.
The emitter structure 3 is often pointed in shape in order to create a region of enhanced electric field at the intended emission site. The gate layer 8 is separated from the substrate 2 by an insulating layer 6, such as, for example, silicon dioxide. Normally, the emission current passes through a first aperture 10 (hereinafter xe2x80x9cgate aperture 10xe2x80x9d) and is collected at a location having a potential of at least several volts more positive than the emission site. The diameter of the gate aperture 10 is typically on the order of 1 micron. To ensure that most of the field emission current passes through the gate layer 8, the gate layer 8 is preferred to have rotational symmetry about a vertical axis, and the emission site 4 is preferred to be located on the axis of symmetry.
In order to make the electric field at the emission site 4 relatively independent of the voltages applied to external electrodes, the exposed face of the gate aperture 10 facing the emitter 3 is preferred to have a thickness similar to the diameter of the gate aperture 10. In some exemplary cases, the emission site 4 is fabricated on a resistive film, creating resistance 14 between the external voltage supply 12 and the emitter 3. The current passing though the resistor 14 creates a voltage opposite the external supply 12, reducing the voltage between the gate layer 8 and emission site 4, thus limiting the emission current.
FIGS. 2 and 3 show exemplary energy distributions measured from typical field emitters as shown in FIG. 1. FIG. 2 is an exemplary energy distribution graph produced by a typical single macroscopic field emitter made from molybdenum wire, operated after being exposed to air. The energy distribution extends to more than 2 eV below the Fermi level (EF), and most of the additional emission current induced by increasing the emitter-gate voltage occurs at the lower part of the energy range. In this example, all of the additional emission measured with gate voltages above 850V occurs at least 1 eV below EF.
FIG. 3 shows an exemplary energy distribution chart created by a typical field emitter array with emitter structures made from n-type silicon. Much of the additional current produced by increasing the gate voltage from 65 to 75 volts occurs at energies more than 2 eV below EF.
Accordingly, a method and apparatus for regulating electron emission in field emitter devices overcoming the above-identified drawbacks is proposed.
A method and apparatus for regulating the emission current from a single (macroscopic) field emitter, from groups of emitters within a large array, or from each cell within an array is described. The apparatus of the present invention includes an additional aperture (filter aperture), fabricated at each field emitter array cell, to create an electron energy filter. The filter aperture of the electron energy filter is preferably similar to the gate aperture but located above or in front of the gate aperture, and is held at a positive potential that is lower than the potential applied to the gate.
The combination of the filter aperture and the filter electrode (referred to herein as an xe2x80x9cenergy filterxe2x80x9d) allows only those electrons with energy greater than a predetermined minimum (the cutoff energy) to pass through. A current-limiting circuit is placed in series with the gate aperture, limiting the total current of electrons that do not pass through the energy filter. Thus, emission from low energies is limited without limiting emission from energies near the Fermi level.
The cutoff energy (measured with respect to the Fermi level (EF) of the substrate contact) is approximately equal to the voltage applied between the substrate and the filter aperture, minus the work function of the filter aperture. The physical dimensions of the filter aperture and adjacent electrodes determine the filter function. The filter function should ideally have a nearly abrupt step from fully transparent to fully opaque at the cutoff energy. In practice, the transition from transparent to opaque can occur in about 1 eV. The cutoff energy can be adjusted by changing the voltage applied to the energy filter. Electrons with energy that is too low to pass through the filter aperture are rejected and are reflected back to be collected by the gate electrode.
An electrical circuit connected to the gate aperture is used to reduce the voltage applied to the gate aperture until the gate current falls below an acceptable level. In this way, only the current which is emitted at excessively low energies is limited. However, the emission will not be limited if it occurs above the cutoff energy. In most cases the current emitted at low energies will naturally increase as the gate voltage is increased, such that the energy filter will begin to reject some of the current.
In cases where failures occur as a result of excessive low energy emission, the low energy emission can be reduced, thereby preventing such failures. In cases where a constant proportion of the emission occurs at low energies, the total emission current from each cell may be regulated to a constant value. These functions may be performed individually for each cell in the array. Alternatively, current limiting circuits can be connected to groups of cells within the array, to the entire array, or to combinations of individual cells, groups of cells, and the entire array.
A small resistance or other circuit element may be placed in series with the emitters to artificially increase the emission energy dispersion with emission current, thereby enhancing the functionality of the energy filter. This approach would be useful in cases where the energy filter function is too broad to detect the natural emission energy dispersion with emission current. The voltage developed across the series resistance should preferably be at least as large as the range of the filter cutoff energy, which might be approximately 1V. This is a smaller voltage than would typically be required to regulate the emission without the energy filter. The resistor may be fabricated in the form of a thin resistive film or resistive post.
Thin resistor films oriented perpendicular to the direction of current flow may be preferred in applications requiring high frequency emission modulation, as they form a capacitance in parallel with the resistance. The parallel capacitance reduces the impedance of the circuit at high frequencies. Such a structure is useful as it enables the displacement current associated with high frequency modulation of the electric field at the emitter surface to exceed the emission current.
The emission energy distribution may be determined by measuring emission current while varying the filter cutoff energy. This may be useful in cases where the emission energy distribution changes over time as a result of changes in the properties of the emitting surface.
No resistors or other circuits need preferably be connected to the filter aperture, hence the filter energy can be modulated at high frequencies. Changing the filter voltage from just below the minimum emitted energy to just above the maximum emitted energy will modulate all the current allowed to pass through the filter aperture. If the field emitters produce most of the current within a narrow energy range, the modulation voltage applied to the energy filter can also be small. Similarly, if a part of the emission occurs within a narrow energy range near EF, the filter energy can be modulated in a narrow range near EF, allowing a fraction of the total emission current to pass through the filter aperture.
One may describe the filter transconductance associated with modulating the filter voltage, distinct from the gate transconductance associated with modulating the gate voltage. The filter transconductance may be substantially larger than the gate transconductance. Thus, the power required to modulate the current that passes through the filter aperture may be substantially lower than the power required to modulate the emission current by changing the gate voltage. The current diverted to the gate may reduce the gate voltage due to the current limiting circuit, which can further improve the overall transconductance by combining the effect of modulating the filter and gate.
The circuit limiting the gate current can be a simple resistor or a more complex circuit including, for example, capacitors, diodes, or transistors. In an exemplary embodiment, the gate layer can create the resistance by fabricating the gate from a resistive material and patterning it to increase the resistance in series with each cell. In another embodiment, a resistive layer can be formed over the exposed surface of the gate aperture by deposition or chemical reaction.
In yet another embodiment, a circuit including a transistor can be fabricated by placing a semiconducting material at the exposed face of the insulating layer separating the gate from the substrate, such that electrons rejected by the filter will strike the semiconductor surface, thereby inducing conductivity in the semiconductor. Alternatively, the insulating layer(s) separating the substrate and gate, and/or gate and filter may be made entirely from an undoped (resistive) semiconductor so that its exposed face becomes less resistive under electron bombardment. The conducting gate and filter layers may be made from doped (conducting) layers in the same semiconducting material.